1. Field of the Invention
The present invention relates to a projection display apparatus for producing on a screen an enlargement of an optical image formed on light valves by modulating lights with the light valves and projecting the modulated lights on the screen by a projection lens.
2. Description of the Prior Art
In order to effect display of images in a large picture size, there has been well known a method of forming on a relatively small light valve an optical image corresponding to an image signal as a variation of optical characteristic, modulating a light with the optical image, and projecting the modulated light on a screen by means of a projection lens. In the projection display apparatus of this type, the resolution of the projected image is nearly determined by the resolution of the light valve, and when the light source is intensified, a larger optical output is obtainable, so that when a light valve of high resolution is used, it becomes possible to realize a projection display apparatus having high resolution and large optical output even if its display area is small. Besides, recently notice has been made on a method of using a liquid crystal panel as a light valve. For example, Morozumi, et. al. proposed in "LCD Full-Color Video Projector", SID 86 Digest, p. 375, a method of obtaining a color projection image using three liquid crystal panels. A basic construction of the optical system proposed by Morozumi, et al. is shown in FIG. 6.
Lamp 1 radiates a light including the color components of red, green, and blue. The light radiated from the lamp 1 is converted into parallel rays by a condenser lens 2 and a concave mirror 3, passed through a heat absorbing filter 4, and then put into a component color separator 5. The component color separator 5 is disposed by placing a flat plate type red reflection dichroic mirror 6 and flat plate type blue reflection dichroic mirrors 7, 8 in X-form intersecting manner, by which the dichroic mirror surfaces 9, 10 lie on the same plane. The light incident on the component color separator 5 is divided into the lights of red, green, and blue. The red light is reflected by the plane mirrors 11, 12 and incident on the red liquid crystal panel 15. The green light is straightly passed and incident on the green liquid crystal panel 16. The blue light is reflected by the plane mirrors 13, 14 and incident on the blue liquid crystal panel 17. On the liquid crystal panels 15, 16, 17 there are formed the optical images of red, green, and blue as the variation of the transmissivity in proportion to the image signals, respectively. The output lights from the liquid crystal panels 15, 16, 17 are combined into a one-piece form by a light combiner 18, and a color image is formed substantially at the position of the green liquid crystal panel, and the color image is projected by enlargement on a screen 80 by means of a single projection lens 19. The light combiner 18 is a prism type dichroic mirror made by joining four rectangular prisms 20, 21, 22, 23, in which the red reflection dichroic multi-layered film is vacuum deposited on the joining surfaces 24, 25 and the blue reflection dichroic multi-layered film on the joining surfaces 26, 27.
The projection display apparatus shown in FIG. 6 has a characteristic to make it possible to change readily the picture size or the distance from the projection lens 19 to the screen 80, because of the use of a single projection lens. Further, since the component color separator 5 and the light combiner 18 cause to cross the dichroic reflection surface in X-letter form, there is a characteristic to reduce the space necessary for the optical system. The construction to intersect a pair of dichroic mirrors in X-letter form as a component color separator or a light combiner has already been disclosed by Schroeder in U.S. Pat. No. 2,642,487 as "component color separator".
By the way, when a pair of dichroic mirrors are intersected in X-letter form, an invalid area is necessarily formed on the intersecting part, and the obscure image of the intersecting part may appear on the screen. Accordingly, the light combiner 18 of the construction as shown in FIG. 6 makes the invalid area of the intersecting part 28 very fine. The intersecting part 29 of the component color separator 5 is also projected in obscure form on the screen, which however does not practically provide a problem if the F-number of the projection lens 19 is small, or if the diameter of the iris of the projection lens 29 is sufficiently large.
The prism type dichroic mirrors to be used as a light combiner 18 for the construction as shown in FIG. 6 are relatively expensive in comparison with those of a flat type, and become more expensive when the invalid area of the intersecting part 28 is made extremely fine. Accordingly, in order to reduce the cost of the apparatus, it is considered to use a flat type dichroic mirror made by intersecting with a light combiner 18 of a construction as shown in FIG. 5 in X-letter form, or to use a prism type dichroic mirror having no fine invalid zone of the intersecting part 28. However, if there are the intersecting parts 28, 29 in front and back of the liquid crystal panel, a bright line is formed at the center of the projected image. This bright line causes a significant problem in deteriorating the image quality.
In view of the fact that no bright line is seen when there is an intersecting part only on the component color separator 5 or the light combiner 18, the mechanism for generating the bright line is distinctly different from that in which the obscure images at the intersecting parts 28, 29 appear on the projected image. As a result of extensive study, the inventors have found out that the cause of this bright line could be explained, as follows:
A state of the case where parallel rays are incident on the component color separator 5 as shown in FIG. 6 is shown in FIG. 7. In this case, it is assumed that the thicknesses of the dichoic mirrors 6, 7, 8 are all equal. The inputted light 30 is dissolved into the rays 31, 32, 33 which advance in division into three directions by the dichroic mirrors 6, 7, 8. Due to the edge surfaces 34, 35 with which the small type dichroic mirrors 7, 8 are in close contact, there are produced the invalid areas 36, 37, 38 on the three output rays 31, 32, 33. These invalid areas 36, 37, 38 are seen as dark zone when observed through the projection lens 19. It is worth noticing that, when the thicknesses of the dichroic mirrors 6, 7, 8 are all equal, the width d.sub.G of the invalid area 37 of the light emitted in the direction of straight advance becomes two-fold the widths d.sub.R, d.sub.B of the invalid areas 36, 38 of the light emitting in the width-wise direction.
Then, when the inside of the projection lens 19 is observed from the position near the center 81 of the screen 80, there are seen the two dark zones 40, 41 corresponding to the two intersecting parts 28, 29 in the iris 39, as shown in FIG. 8. The two dark zones 40, 41 are seen in overlapped state when observed from the center 81 of the screen 80, and they shift in the opposite directions to each other when the position of the eye is moved. Under the state where the two dark zones 40, 41 do not overlap, the total area of the dark zones becomes approximately the same without respect to the distance thereof, but under the state of overlap with the condition of contact between the two dark zones 40, 41, there are great differences in the total areas of the dark zone. When the eye position is moved in the vicinity of the center 81 of the screen 80, the size of the iris 39 is approximately the same, and only the total range of the dark zones sharply varies. Since the illuminance on the screen 80 is proportionate to the amount obtained by substracting the total area of the dark zone from the area of the iris 39, there appears sharp change in the illuminance near the center of the screen, which is seen as a bright line.
The bright line is prominently seen with the green light, but it scarely matters with the rays of red and blue. This is attributed to the fact that the optical path length from the intersecting part 29 of the component color separator 5 to each of the liquid crystal panels 15, 16, 17 is short in the green light, and long in the red and blue lights. That is to say, it is because of the fact that, when the dark zones of red and blue lights are observed from the area near the center 81 of the screen 80, the dark zones of the red and blue lights are seen at a distance farther than the dark zone of the green light, and for that part the dark zone of the red and blue lights are seen finer.
It is seen from the above that, in order to make the bright line less prominent, it is better to make at least one dark zone finer than the other dark zone. Thus, it becomes impossible to use a flat plate type dichroic mirror made by intersecting in X-letter form with both the component color separator 5 and the light combiner 18, or a prism type dichroic mirror in which the invalid area of the intersecting part is not fine. In order to make one of the two dark zones 40, 41 finer, it is conceivable to make at least one of the component color separator 5 or a light combiner 18 into extremely thin flat type dichroic mirror, but such dichroic mirror involves many problems from the points of mechanical strength and processability.